Reference is made to related application Ser. No. 08/001,702 entitled "Base Metal Only Catalyst System for Lean Burn Engines" filed Jan. 7, 1993 by the present inventors, which application is commonly assigned with this invention.
This invention is directed to a two-stage catalyst system comprising a first-stage nitric oxide removal catalyst and a second-stage carbon monoxide and hydrocarbon removal catalyst for treating the exhaust gases produced by an internal combustion engine.
A number of catalysts have been suggested to convert engine exhaust gas components like carbon monoxide (CO), hydrocarbons (HC's), and nitrogen oxides (NO.sub.x) into other gases. The first two are desirably oxidized to H.sub.2 O and CO.sub.2 while the nitrogen oxides present in the exhaust gas, generally nitric oxide, are desirably reduced to N.sub.2. These so called "three-way" catalysts achieve simultaneous efficient (conversion &gt;80%) removal of CO, HC, and NO.sub.x when the fuel mixture of an internal combustion engine is slightly "rich" in fuel, i.e., in a narrow A/F ratio range between about 14.7 and 14.4, and the exhaust gas is slightly reducing. Such three-way catalysts are not efficient, however, in the reduction of NO.sub.x when engines are operated on the lean (reduced fuel) side where the A/F ratio is greater than 14.7, generally 19-27, and the exhaust gas is richer in oxygen. It is desirable, however, to operate engines on the lean side to realize a benefit in fuel economy, estimated to be in the range of 6-10 %.
In addition to three-way catalysts, two-stage conversion systems have also been proposed for treating exhausts and involve an initial contacting zone directed to removal of NO.sub.x and a second contacting zone directed to removal of CO and HC's.
Gladden in U.S. Pat. No. 4,188,364 discloses a system wherein the nitric oxide content is reduced through a reaction with ammonia in the first catalyst bed comprising a porous inorganic oxide. The gas stream, containing oxygen, ammonia and reduced nitric oxide content is subsequently contacted with a second catalyst bed having an oxidation catalyst disposed on a porous inorganic oxide carrier, comprising a noble metal or other metals such as copper, zinc, or tungsten. The resultant exhaust stream is disclosed to be substantially nitric oxide and ammonia free. Gladden's invention is not suitable for automotive application, however, because this system requires the storage of ammonia on board a vehicle.
Gandhi et al. in U.S. Pat. No. 4,374,103 disclose a catalyst system useful in fuel-rich applications in which the exhaust gases initially flow over a catalyst comprising palladium and subsequently over a catalyst comprising palladium-deposited on tungsten. The first catalyst bed operates slightly rich of stoichiometry. Since the engine is required to operate fuel-rich to provide reducing conditions at the inlet of the catalyst, fuel economy is adversely affected. Also, tungsten is present on the support in large amounts, generally around 50% by weight of alumina, in the second-stage catalyst.
It would be desirable to have a catalyst system which would be effective in reducing nitric oxide emissions and also provide high conversions for hydrocarbons and carbon monoxide under lean-burn conditions (oxygen rich exhaust situations). Such a system would allow for improved fuel economy. In lean burn situations, considerable success has been achieved in the catalytic oxidation of unburned hydrocarbons and carbon monoxide, but the reduction of the nitrogen oxides has proven to be a much more difficult problem. This is because the reducing substances (such as CO or H.sub.2) tend to react more quickly with the oxygen present in the exhaust gas than with the oxygen associated with nitrogen in NO.sub.x. The present invention overcomes such problems.